1. Field of the Invention
The present invention relates to an electronic ballast for the operation of at least one low-pressure discharge lamp.
2. Description of the Related Art
Usually, nowadays ballasts are used that supply a high-frequency alternating voltage to the gas discharge lamps or fluorescent tubes. Apart from the voltage supply, such electronic ballasts are used, moreover, to preheat the electrodes of the gas discharge lamps and to ignite and operate the lamps gently. With the aid thereof, the degree of efficiency of the lamps is increased, a longer service life is attained and operation with reduced lamp output (dimming) is also rendered possible.
In this connection, before the igniting voltage is applied to the discharge lamp, the electrodes or the coils of the lamp are as a rule preheated for a specific period of time, thereby attaining a comparatively gentle start of the lamp and thus a longer service life of the lamp. Preheating is effected with the aid of coil-heating that brings about a flow of current through the two coils. In a ballast that is known from EP 0 707 438 A3, for this a heating transformer is used, the primary winding of which transformer is connected to the output of an inverter and which transformer has two secondary windings which are each coupled to one of the two lamp coils. Before the ignition of the discharge lamp, a frequency is set for the alternating voltage supplied by the inverter, which frequency is varied in relation to the resonant frequency of the series resonant circuit in such a way that the voltage that is applied to the discharge lamp does not, first of all, bring about any ignition of the lamp. Meanwhile, a substantially constant current flows through the two secondary heating circuits with the lamp coils, whereby the latter are preheated. After a period of time that suffices for preheating, the frequency of the alternating voltage that is fed to the series resonant circuit is then shifted in the direction of the resonant frequency for so long until the thereby increasing voltage that is applied to the discharge lamp brings about ignition of the lamp. According to EP 0 748 146 A1 or DE 295 14 817 U1, by opening a switch that is connected in series with the primary winding, the coil-heating can be switched off after the lamp has been ignited in order to reduce power losses that would otherwise occur.
The demands on the electronic ballasts in this connection are becoming more and more extensive. Thus, for example, it is usual for a dimming operation to be provided for the gas discharge lamp as well. Dimming that is effected to a great extent would, however, result in the lamp electrodes cooling below their emission temperature and thus in premature ageing of the lamp. In order to counteract this effect, the electrodes of the gas discharge lamp also need to be heated to a certain degree during the operation in which ignition has already taken place. In particular, it is advantageous to set the heating of the electrodes as a function of the degree of dimming in such a way that the latter are heated all the more, the greater the lamp is dimmed, that is, the darker it is. In accordance with EP 0 707 438 A2, the heating of the electrodes is regulated during the dimming in that the switch that is connected in series with the primary winding is closed for a short time.
The ballast should in addition also take on a monitoring function monitoring the state of the lamp in order to be able to detect possible operational disturbances and introduce appropriate measures. An operational disturbance can, for example, exist if one of the two coils or even both is or are defective or if the lamp has been completely removed. In the case of the electronic ballast described in EP 0 707 438 A2, the voltage drop across a resistor that is connected in series with the primary winding of the transformer and thus the heating current are measured in order to detect whether there is a coil-break or whether the lamp has been removed from the arrangement.
The method that has just been mentioned provides information on the state of the lamp, but not on what type of lamp it is. Often lamps do not differ externally, yet have differing electrical parameters and different power consumption. If a lamp whose performance features do not suit the electronic ballast is used in error, activation errors can result. In comparatively simple cases this impairs the illumination, but in more serious cases it can also result in damage to the lamp. Such problems could be avoided by detecting the type of lamp before ignition in a short check measurement and introducing appropriate measures. This can mean that the lamp is not preheated and ignited if it is the wrong type or better still that activation is effected that corresponds to the performance features of the lamp.
Basing considerations on the prior art mentioned above, it is therefore an object of the present invention to specify an electronic ballast for operating a low-pressure gas discharge lamp that performs the functions which have just been described, that is, lamp-identification, detection of the state of the lamp and coil-heating with controllable output, with the least possible outlay in terms of material and circuitry.
This object is achieved by means of a ballast which has the features of the present invention. An important feature of the ballast is an evaluating circuit arrangement which, for the purpose of identifying the type and the state of the lamp, detects and evaluates the current flowing through the primary winding of the heating transformer and in addition also the current flowing through at least one of the two heating circuits. The type of lamp is then identified by measuring the current that flows by way of the lamp coil and which represents a suitable measure of the coil resistance. The coil resistance in turn is a characteristic feature for distinguishing between lamps that have the same appearance, but different performance features. The current through the primary winding, on the other hand, provides information on the state of the lamp. The transformer steps down the heating voltage at the primary winding towards the lamp to a great extent so that the levels of coil resistance, for their part, are stepped up towards the primary winding. The behaviour of the transformer therefore depends greatly upon whether the coils are intact or whether, for example, a coil is defective and thus the pertinent secondary heating circuit is interrupted.
Furthermore, it is an object of the invention to render possible optimum control of the heating current and thus of the coil-heating. This is achieved in accordance with more specific features of the invention in that the connection of the primary winding of the heating transformer to the output of the inverter is regulated by a bidirectional switch consisting of two switches, with the primary winding of the heating transformer and a coupling capacitor being arranged between the two switches. The bidirectional switch can be formed by means of two field-effect transistors that are connected in series and are orientated in opposition to each other and are preferably activated by means of a common pulse-width modulated signal, with the pulse duty factor of this signal determining the degree of heating. With the aid of this arrangement, temporary discharge of a coupling capacitor contained in the heating circuit is avoided and thus a symmetrical heating voltage is attained.
Further developments of the invention constitute subject matter of the subclaims. The resistance value of one of the two coils is used in order to determine the lamp typexe2x80x94as has already been mentioned. The latter is determined by way of the peak value of the so-called pin current. In order to identify a coil-break or removal of the lamp, the current at the primary winding and at the same time as well the pin current are measured and both currents are set in relation to each other. This method makes it possible to make a statement on the state of the lamp independently of possible voltage fluctuations. In this connection, it is preferably first examined whether an intact lamp is present and only subsequently is the lamp type determined. In order to increase the reliability of the determination of the lamp, the measurement can be carried out twice, once before and once after the preheating of the lamp. The resistance values thereby measured can be compared with internally stored reference values and can then be associated with known lamp types. Furthermore, before the start of the coil-preheating and the lamp-identification a short test can be carried out to determine whether the coils are also actually cold. in this way, misinterpretations in the identification of the lamp, which can occur after a short-term mains failure, can be avoided. The current-measurements are preferably effected in each case by measuring the voltage drops across two measuring resistors which are arranged in the heating circuit of the primary winding and in the secondary heating circuit of a lamp coil respectively.
In a further development of the invention, the electronic ballast is constructed in such a way that the coil-heating is activated and the frequency of the alternating voltage that is applied to the load circuit with the lamp is set as a function of the type of lamp previously determined. In order to set the coil-heating as a function of the degree of dimming of the lamp, it can be established that the lowering of the lamp current brought about by the dimming of the lamp is to be substantially compensated for by the heating current. The level of the rated value for the pin current, that is, for the sum of the lamp current and heating current, is, in this connection, determined by the electronic parameters of the lamp. Preferably as well after the ignition of the lamp, a check measurement is carried out at regular intervals in order to identify a coil-break that might possibly have occurred or to identify removal of the lamp.
The invention shall be explained in greater detail in the following with reference to the enclosed drawing in which: